(1) Field of the Invention
The present invention relates to ribbon tensioning devices and more particularly to a releasable ribbon locking device operable in a ribbon cartridge for maintaining an exposed spanwise portion of the ribbon in a taut or non-sagging condition.
(2) Description of the Prior Art
In the field of ribbon tensioning devices contained in a removable ribbon cartridge, it is a primary concern that the ribbon remain in a relatively taut condition at all times whether the cartridge is operatively positioned in a printing machine or removed therefrom, say for storage. A taut ribbon provides a true straight-line portion of the ribbon in spanning a print gap of the cartridge that exposes ribbon external from a protective housing. Thus, cartridges equipped with a ribbon tensioning device are easily and quickly interchangeable without concern that the exposed portion of the ribbon may snag or rub members in the printing machine such as a printing element, a print indicator or a ribbon guide located near the cartridge changing path. Also, in using the cartridge in the machine, the ribbon should remain taut so as to avoid contact with proximal members during vibratory motion, minimize the possibility of the ribbon becoming entangled in the cartridge and enable a ribbon feed mechanism to accurately advance the ribbon the prescribed amount.
Heretofore many ribbon cartridges employing a form of a ribbon tensioning device have been developed. The prior art fails to disclose a ribbon tensioning device that satisfies all the conditions for keeping a ribbon continually taut. For example, U.S. Pat. No. 4,074,799, granted Feb. 21, 1978 to Hishida et al discloses a ribbon cartridge with a ribbon slack prevention device that is only operable for keeping the ribbon taut when the cartridge is removed from the typewriter. Another example of a ribbon cartridge having a ribbon tensioning device is U.S. Pat. No. 4,010,839 granted Mar. 8, 1977 to Guerrini et al. In Guerrini et al, the disclosed ribbon tensioning device operates using a detent pin that engages notches on a feed spool in order to prevent chance unwinding of the ribbon. The problem in using the detenting system for ribbon tensioning purposes is that the operative range for properly holding the ribbon is restricted to rotation of the feed spool in amounts defined by the fixed notched spacing. It can be appreciated by one skilled in the art that rotation of the feed spool for a given ribbon advancement will vary in amounts depending on the diameter of the ribbon wound thereon. Accordingly, the detent pin is not precisely aligned with a notch for all ribbon feed advancements and as a result affects the ribbon tautness.